Quantitative investigations of phenolic compounds associated with root rot of hydroponic pepper (Capsicum annuum L.) caused by Pythium aphanidermatum (Edson) Fitzp

The role of phenolic compounds in epidemics of root rot caused by Pythium aphanidermatum (Edson) Fitzp. in hydroponic pepper (Capsicum annuum L.) was investigated. Phenolic compounds that accumulate in the cytoplasm (free phenolics) and on cell walls (bound phenolics) of roots, shoots and nutrient solution of plants were examined in relation to time after inoculation with P. aphanidermatum and the temperature regime used. Biotrophic colonization by P. aphanidermatum in plants maintained at temperatures of 22--19°C yielded phenolic concentrations that were similar to control plant tissues, but phenolics in the nutrient solution were higher than in controls. Transition from biotrophy to necrotrophy was observed when temperatures increased from 22°C to 28°C and phenolic concentrations within the plant and nutrient solution increased significantly over noninoculated controls. Two phenolic compounds isolated from necrotic brown roots and one isolated from nutrient solution of inoculated plants were identified from HPLC and NMR analyses as 4-hydroxybenzoic acid (4-HBA), and 4-methoxy-3-hydroxybenzoic acid (vanillic acid), respectively. Pure 4-HBA and vanillic acid predisposed healthy plants to infection by zoospores of P. aphanidermatum. This is considered to be the first demonstration of host predisposition to infection by P. aphanidermatum caused by phenolic compounds from diseased plants. Additionally, when P. aphanidermatum was grown on water agar amended with each of the compounds, colony diameters were not significantly different from water agar alone. In contrast, numbers of potential zoospore-forming structures were significantly elevated at all concentrations of 4-HBA, or suppressed or not significantly different at some concentrations of vanillic acid compared to controls. This finding that the compounds have little effect on the growth and reproduction of the pathogen is considered in relation to a revised disease and infection cycles of P. aphanidermatum and to improved methods for the control of the pathogen in commercial hydroponic systems.